Consideration of information disclosed by the following U.S. Patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position on of a camshaft relative to a crankshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, P.sub.C, on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end which results from a compression spring that acts thereon.
U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replace the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. No. 5,002,023. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon.
U.S. Pat. Nos. 5,172,659 and 5,184,578 both address the problems of the aforementioned types of VCT systems created by the attempt to balance the hydraulic force exerted against one end of the spool and the mechanical force exerted against the other end. The improved control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool. The hydraulic force on one end results from the directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, P.sub.S. The hydraulic force on the other end of the spool results from an hydraulic cylinder or other force multiplier which acts thereon in response to system hydraulic fluid at reduced pressure, P.sub.C, from a PWM solenoid. Because the force at each of the opposed ends of the spool is hydraulic in origin, based on the same hydraulic fluid, changes in pressure or viscosity of the hydraulic fluid will be self-negating, and will not affect the centered or null position of the spool. There are, however, several disadvantages inherent in the inventions disclosed in the '659 and '578 patents.
One disadvantage is the inability of this differential pressure control system ("DPCS") to properly control the position of the spool during the initial start-up phase of the engine. When the engine first starts, it takes several seconds for oil pressure to develop. During that time, the position of the spool valve is unknown. Because the system logic has no known quantity with which to perform the necessary calculations, the DPCS is prevented from effectively controlling the spool valve position until the engine reaches normal operating speed.
Another problem with existing VCT systems is sluggish dynamic response. Even after the engine stabilizes at normal operating speed, individual characteristics vary substantially from engine to engine. A new engine at high speed and low temperature can have a drastically different oil pressure than a worn engine at hot idle. Current methods employed to allow operation over such a wide spectrum of engine characteristics (such as increased cross-sectional area of the hydraulic piston and the undersizing of springs) result in a slow response time, requiring relatively low closed-loop controller gains to maintain stability.
The low closed-loop controller gains render the system more sensitive to component tolerances and operating environment. The net effects (such as a change in the PWM duty cycle required to achieve a null position of the spool) cause degradation of overall closed-loop system performance.
Finally, the moving parts of the PWM solenoid typically used in a conventional DPCS create unwanted noise in the system. During operation, the solenoid cycles through its full stroke with every PWM pulse. The rapid cycling results in armature and poppet "chatter", i.e., high frequency collisions, thus introducing the unwanted noise.